1. Field of the Invention
The present invention relates generally to bubble jet printing systems that use the volume change of bubbles produced by heat to spray small jets of conductive ink on plain paper. More particularly, this invention is concerned with an ink-jet printer of the excitation type wherein a current is passed through a conductive ink to cause the ink to become vaporized and cause any trapped gases or bubbles to expand, forcing droplets of ink to jet onto the surface of a material to be printed upon.
2. Description of the Prior Art
A conventional ink-jet printer includes, as shown here in FIG. 4, an ink-jet head 1 having an ink tank 3 for holding therein a conductive ink 2, and a nozzle 4 connected to an end of the ink tank 3. The nozzle 4 is open at an ink tank side and closed at the opposite side. The ink tank 3 and the nozzle 4 communicate with each other via an ink passage 5, so that the conductive ink 2 held in the ink tank 3 is allowed to flow through the ink passage 5 into the nozzle 4. The nozzle 4 has in its peripheral wall a discharge hole 6 from which the conductive ink is ejected in the form of droplets. A pair of electrodes 7 and 8 is disposed on an inside wall of the nozzle 4 at a position diametrically opposite to the discharge hole 6. To the electrodes 7, 8, a voltage is applied by an electrode exciting device or unit 9 which is controlled by an output signal sent from a central processing unit (CPU) 10. Numeral 11 is a power supply for supplying electrical power to the electrode exciting unit 9, and numeral 12 is a current flowing through the conductive ink 2 when the voltage is applied across the electrodes 7 and 8.
The conventional ink-jet printer of the foregoing construction operates as follows.
When the CPU 10 sends a low level signal to the electrode exciting unit 9, the electrode exciting unit 9 is in an inoperative or "off" state and the ink-jet printer is in the stand-by condition. When the CPU 10 sends a high level signal to the electrode exciting device 8, the electrode exciting unit 9 is operated or turned on whereupon a voltage from the power supply 11 is applied across the electrodes 7 and 8. Upon application of the voltage to these electrodes 7, 8, a current 12 is passed through the conductive ink 2 contained between the electrodes 7, 8, causing the generation of heat which in turn will vaporize that portion of the conductive ink 2 contained between the electrodes 7 and 8. Gases or bubbles produced on vaporization expand suddenly, exerting a sufficient pressure upon the conductive ink 2 to force the conductive ink 2 to eject from the discharge hole 6 of the nozzle 4 to the surface of a material to be printed upon.
Thereafter, the electrode exciting unit 9 is turned off or de-energized whereupon the current 12 flowing between the electrodes 7 and 8 disappears. Consequently, heat of the bubbles produced in the conductive ink 2 is immediately taken up by the surrounding conductive ink 2 and the bubbles disappear soon. Thus, the ink-jet printer is returned to the stand-by condition.
According to the foregoing construction, due to electrolytic corrosion and cavitation caused by repeated generation and disappearance of the bubbles, the electrodes 7 and 8 wear down gradually with the result that the distance between the electrodes 7 and 8 increases progressively. As the inter-electrode distance increases, the current 12 flowing through the conductive ink 2 contained between the electrodes 7 and 8 decreases. With this reduction of the current 12, only an insufficient heat energy can be produced in order to vaporize the conductive ink 2. Under such condition, a stable spouting of the conductive ink 2 is no longer possible. If printing operation continues with such unstable spouting of ink. the printing quality is significantly deteriorated.